How to use Shader Nodetree in emissions rendering?
In bpy.data.node_groups you will find node groups that you have made, that is nodes that you have turned into a group that you can use in multiple materials.

The nodes for a light can be found in the lights data property.

light_nodes = bpy.data.objects['lamp1'].data.node_tree.nodes
light_nodes['Emission'].inputs['Strength'].default_value = 10000